Towards high-efficiency GaAs thin-film solar cells grown via close space vapor transport from a solid source

Abstract

GaAs is an attractive photovoltaic material, but its widespread implementation is limited in part by the high cost of metal-organic chemical vapor deposition, which employs toxic and pyrophoric gas-phase precursors. We study close-space vapor transport, which uses solid GaAs as a source and water vapor as a transport agent as an alternative technique for depositing GaAs films. Epitaxial n-GaAs thin films were grown on n + -GaAs substrates while varying the water vapor concentration and the source/substrate temperatures. The photovoltaic properties of the films were evaluated using a non-aqueous photoelectrochemical test cell containing the ferrocene/ferrocenium redox couple. We found that a wide range of growth conditions were suitable for growing GaAs films with diffusion lengths over 1 µm and open circuit voltages similar to what has been observed for GaAs grown using metal-organic chemical vapor deposition (~ 820 mV).

Analysis of performance-limiting defects in pn junction GaAs solar cells grown by water-mediated close-spaced vapor transport epitaxy

“…However, while O has been detected in CSVT films at a concentration ~10 16 cm -3 by SIMS [26], the midgap state has only been detected by deep-level transient spectroscopy (DLTS) when O 2 was intentionally introduced into the reactor [27]. Increasing water vapor concentrations during growth have also been correlated with degraded photoluminescence [28] [4], although in our own work with electrochemical cells J sc was only degraded at very high water concentrations (>4000 ppm) [20]. The influence of oxygen on device performance merits further study.…”

“…We have reported that the electronic properties of GaAs grown by CSVT using H 2 O transport are satisfactory for high-efficiency PV devices, and have demonstrated the first working homojunction GaAs solar cells [19][20][21][22]. Here, we report on progress towards high-efficiency CSVT GaAs solar cells.…”

“…The best devices for the wafer absorber sample in Figure 5B have higher V oc than the best CSVT absorber devices in Figure 5A due to the lower number of surface defects in that sample, and therefore a lower likelihood of shunting. Devices with CSVT absorbers have a higher current since the minority carrier diffusion lengths are higher in CSVT material than in the wafer substrates (which are produced by the vertical gradient freeze technique) [19,20]. Optical images of two defect sites analyzed by ToF-SIMS are shown in Figure 6.…”

Homojunction GaAs solar cells grown by close space vapor transport

“…arsine and trimethylgallium) employed by metal-organic chemical vapor deposition (MOCVD). Close space vapor transport (CSVT) of GaAs is a plausibly scalable process, similar to commercial CdTe deposition, which uses water vapor to generate gasphase As 2 and Ga 2 O in-situ at atmospheric pressure with high (~1 µm/min) growth rates and ~95% overall transport efficiency [1] - [2].…”

“…The reactor used for growth has been described in detail in previous publications [1] - [2]. The reaction 2 GaAs + H 2 O (g) Ga 2 O (g) + As 2 (g) + H 2 (g) occurs at atmospheric pressure under an H 2 ambient at temperatures typically between 700 and 850 ˚C.…”